1. Field of the Invention
The present invention relates to a blower fan and more specifically to a blower fan preferably for use in cooling an electronic device or the like.
2. Description of the Related Art
As a motor used to drive a blower fan, an outer-rotor motor with a rotor arranged outside of a stator, has been primarily used with the view of facilitating an assembly process, reducing the number of parts, and so on. The outer-rotor motor is also favorable in terms of performance because the outer-rotor motor has a large moment of inertia and is capable of generating a large driving torque, and therefore has an excellent capability to rotate at a constant speed.
Meanwhile, electronic devices have been experiencing an increase in density of their components in recent years, and in accordance with this increase in density, the amount of heat generated by the electronic devices has been increasing. Accordingly, higher rotational speeds have been demanded of blower fans used to cool the electronic devices. However, because of the large moment of inertia, the outer-rotor motor tends to experience increased vibrations with increased rotational speed of the fan. The outer-rotor motor may therefore experience a problem in terms of strength.
An inner-rotor motor, in which a rotor is arranged inside of a stator, has a smaller moment of inertia than the outer-rotor motor. Therefore, use of the inner-rotor motor enables an increase in the rotational speed of the fan without a significant increase in vibrations.
For example, US 2009/0180901 describes a blower fan in which an inner-rotor motor is used. This blower fan includes a motor support portion that includes a bearing support portion arranged to support bearings and a stator support portion arranged to support a stator. These support portions are integral with each other.
In this blower fan, the bearing support portion, a rotor holder (including a rotor magnet), the stator, the stator support portion, an impeller cup, and blades are arranged in this order from a rotation axis toward a radial outside. In addition, radial gaps of specified widths are defined between the bearing support portion and the rotor holder, between the rotor holder and the stator, and between the stator support portion and the impeller cup.
On the other hand, in a typical blower fan of an outer-rotor type, a bearing support portion, a stator, a rotor holder (including a rotor magnet), an impeller cup, and blades are arranged in this order from a rotation axis toward a radial outside. In addition, the bearing support portion serves also as a stator support portion, and the impeller cup is press fitted to an outer circumference of the rotor holder. Therefore, a radial gap of a specified width is defined only between the stator and the rotor holder. A blower fan of an inner-rotor type therefore has a larger number of components arranged in a radial direction than the blower fan of the outer-rotor type, and therefore, a motor portion of the blower fan of the inner-rotor type has an increased radial dimension.
JP-A 2006-322451 describes a blower fan of the inner-rotor type (hereinafter referred to as a “known blower fan”). In the known blower fan, a rotor magnet is fixed to a shaft, and a motor portion is spaced axially away from an impeller and a bearing support portion. In the blower fan of the inner-rotor type having the above-described structure, the rotor magnet, a stator, and a stator support portion are arranged in this order from a rotation axis toward a radially outside portion of the blower fan. In addition, a radial gap of a specified width is defined only between the rotor magnet and the stator. Accordingly, a reduction in radial dimension of the motor portion is achieved.
Furthermore, in the known blower fan, the shaft is supported by the bearing support portion, and each of the impeller and the rotor magnet is joined to the shaft such that the impeller and the rotor magnet are arranged on an axially upper side and an axially lower side, respectively, of the bearing support portion. Therefore, correction of an unbalance that results from a displaced center of gravity of a rotating body including the impeller, the shaft, and the rotor magnet needs to be carried out in a situation in which the rotating body has been defined in one united body. In other words, correction of an unbalance of the rotating body needs to be carried out after the rotating body including the impeller, the shaft, and the rotor magnet is assembled. In view of a moment of inertia, it is desirable that the unbalance correction should be performed at a position near the center of gravity of the rotating body and as radially distant from a shaft center as possible. The amount of the correction can thus be minimized. Therefore, the unbalance correction is normally performed at an opening end portion of the impeller cup.
The impeller and the stator support portion, which is substantially cylindrical in shape, are arranged axially opposite each other. In order to reduce ventilation resistance of a wind tunnel portion, it is desirable that the outside diameter of the impeller cup and the outside diameter of the stator support portion be substantially equal to each other. Moreover, in order to reduce resistance against a wind blowing around an outer circumference of the stator support portion, it is desirable to reduce the distance between the impeller cup and the stator support portion.
However, the above-described desirable arrangements mean that the opening end portion of the impeller cup is arranged so close to the stator support portion that a sufficient space for the unbalance correction cannot be secured, making it difficult to perform the unbalance correction after assembling of the blower fan.
Furthermore, regarding the known blower fan, the rotating body including the impeller, the shaft, and the rotor magnet has a large axial dimension, in addition to having a small moment of inertia, and therefore tends to easily experience a displaced center of gravity due to assembling errors. Therefore, insufficient correction of the unbalance of the rotating body would lead to more marked occurrence of vibrations owing to the unbalance when the blower fan is caused to rotate at a high speed.